Independent investigators issued a scathing report Sunday on the disappearance of 43 Mexican students, accusing the government of obstructing their probe and questioning the role of federal forces in the tragedy.

After a year-long investigation ending this month, the foreign experts from the Inter-American Commission on Human Rights were unable to resolve a case that has shocked the international community and sparked protests against President Enrique Pena Nieto.

While the mystery remains, the report calls for investigations into the conduct of federal police and the military on the night of September 26-27, 2014, when the 43 young men vanished in the city of Iguala, southern Guerrero state.

The experts also invoked allegations of torture against as many as 17 of the more than 100 suspects detained in the case.

But part of the 605-page report -- the second of their mission -- is dedicated to the "obstructions" that the experts faced from the authorities and which became worse from January.

The authorities showed "little interest" in moving forward with new lines of investigation and it was "impossible" for the experts to reinterview 17 suspects in prison, the report said.

"The group has also suffered a (media) campaign that seeks to discredit people as a way to question their work," said the report by the five-member panel -- two lawyers from Colombia, another from Chile, a former attorney general of Guatemala and a Spanish psychologist.

The government has said that it cooperated with the group, giving them unusual access and enough time to investigate.

The experts arrived in Mexico in March 2015 at the request of the victims' parents and after the government agreed to their mission. Their mandate was renewed once, but the authorities decided against giving them another extension.

Prosecutors say the students were attacked by municipal police after the young men stole five buses that they planned to use for a future protest. Three students and three bystanders were killed on the spot.

The officers then handed over 43 students to the Guerreros Unidos drug gang, which killed them and incinerated their bodies at a garbage dump in the nearby town of Cocula, according to the government's account.

The remains of only one student were fully identified after they were found in a nearby river.

The report does not say that federal forces were directly involved in the mass disappearance, but it notes that they were present that night.

The experts repeatedly sought to interview 27 members of the 500-strong 27th army battalion based in Iguala, without success.

The report noted that the army monitored the students' movements through a regional command center and that a military intelligence officer photographed a clash between students and Iguala police.

The experts called on the authorities to investigate allegations that one battalion soldier, nicknamed "The Satanic One," trafficked weapons for the Guerreros Unidos.

Another "key element" that needs further investigation is the "participation or knowledge" of federal police in the mass disappearance.

Students in one of the five buses said federal police pointed their guns at them as they exited the vehicle, prompting them to run.

At a city exit, federal police manned a checkpoint and a detour, according to survivors from an attack on a bus carrying a third-division football team.

Farther down the road, the team's bus was shot at by masked gunmen and people in police uniforms. When federal police showed up after the attack, witnesses say they failed to help the wounded.

The report also recalls allegations that federal police were present near a judicial building where a group of students was attacked and detained by Iguala officers.

The motive in the case remains unclear.

Authorities have said that the mayor of Iguala, who was arrested, ordered police to confront the students over fears they would disrupt a political event for his wife. Prosecutors also said the Guerreros Unidos confused the students for a rival gang.

But the experts, who rejected several allegations in their first report in September, reiterated that the students never planned to disrupt the political event and were not gang members, as some suspects claimed.

They again rejected the government's central conclusion, reiterating on Sunday that there was no scientific proof that an immense fire took place at the landfill.

They also insisted in the new report that the authorities should investigate whether the students were attacked because they may have inadvertently taken a bus that was used by the gang to smuggle heroin.

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